One primary concern about how to backup is whether the system has UFS or ZFS filesystems. You can research how to backup each on this forum and/or the web. Basically, the backup method is the same whether you are backing up over NFS, tape drive, USB drive, or whatever. You simply need to be able to specify a 'device' to write to.
Also, in the case of USB external disks, you need to know how to write a filesystem to the device and then mount it to make it available. Start by reading this thread here:
How to backup (create image) SunOS 5.10 sparc?
where I give advice on USB disk specifically. I also refer to a previous thread about backup in general. Follow those links too.